A comparison of oviparous and viviparous pea aphid development during oogenesis and early embryogenesis.

摘要

The pea aphid (Acyrthosiphon pisum) is a valuable model for evolutionary developmental biology, in part, because of its two highly divergent modes of development. In the spring and summer, aphids undergo parthenogenetic, viviparous development; in the fall, a reduced photoperiod induces the production of males and sexual females whose progeny develop as overwintering eggs. The oviparous and viviparous developments differ in location, maturation time and embryo size, yet they produce nearly identical nymphs.; Early developmental patterning in aphids is complicated by the presence of bacteriocytes, a novel aphid cell type containing the aphid's obligate symbiotic bacteria, Buchnera aphidicola. Buchnera is transmitted from mother to offspring, but the details of the transfer differ in the two developments.; I have used a candidate gene approach to understand Acyrthosiphon oogenesis and early embryogenesis. In Chapter 1, I use the earliest identified bacteriocyte specific gene, Acyrthosiphon Distal-less (Ap-DII), to examine bacteriocyte formation. In oviparous development, Ap-DII is expressed in future bacteriocytes when they associate with Buchnera in the posterior of the egg. In viviparous development, Ap-DII is expressed in the posterior nuclei of the embryo before Buchnera enters. Ap-DII also has evolutionary conserved expression in the distal portion of developing appendages.; In Chapter 2, I test the hypothesis that the presence of bacteriocytes causes the usually posterior germ cells to be specified at an alternative location. I used 2 germ cell marker to track germ cells. Acyrthosiphon nanos mRNA and vasa protein suggests that, at blastoderm, germ cells are specified in a posterior ring. Surprisingly, Acyrthosiphon contains two vasa genes, neither of which express mRNA in germ cells.; Finally, to further investigate posterior embryonic patterning, in Chapter 3, I examine components of the torso-signaling pathway. Acyrthosiphon torso-like (Ap-tsl), a determinate of torso signaling, is expressed in the posterior follicle cells during oviparous oogenesis. Diphosphorylated ERK is localized to the posterior, most likely as a result of torso signaling. During viviparous oogenesis, Ap-tsl is not expressed. This result is confirmed by the unlocalized diphosphorylated ERK in viviparous oocytes. This suggests that the oviparous and viviparous oocytes are being patterned by different mechanisms.